With recent developments in magnetic recording media, there is a growing desire for a magnetic recording medium which can reproduce images or sounds of good quality. Under these circumstances, improvements in electromagnetic characteristics are being made especially by using finer ferromagnetic particles or heightening the loading of ferromagnetic particles. It is also required to diminish noises on carriers to heighten S/N, and this can be attained by making the surfaces of magnetic tapes smoother. Such smoother surfaces result in increased friction coefficients of the magnetic tapes and, hence, it is required to further improve running properties and durability. Further, magnetic recording media should be manufactured at low cost since they are nowadays selling in large quantities. As a means for meeting the above requirements, there is a recording medium having plural superposed magnetic layers, which is advantageous in that proper ferromagnetic particles can be selected so as to impart high-quality image-reproducing properties to the upper magnetic layer and also to impart high-quality sound-reproducing properties to the lower magnetic layer. The above recording medium with plural magnetic layers has another advantage that it can be produced at low cost because materials suitable for the respective magnetic layers can be used. At present, known as means for improving electromagnetic characteristics are to improve the surface properties of magnetic layers and to heighten the loading of ferromagnetic particles in magnetic layers.
In order to increase the loading of ferromagnetic particles in a magnetic layer, a method is employed in which the contents of solid ingredients other than ferromagnetic particles, i.e., a binder, a lubricating agent, an antistatic agent, an abrasive material, etc., are decreased, or a method is employed in which the loading of ferromagnetic particles is heightened by effectively dispersing the ferromagnetic particles by means of a particular dispersing agent or dispersing machine. However, the former method is disadvantageous in that the resulting magnetic tape product is poor in physical properties, particularly in the adhesive strength and tensile strength of the magnetic layer, and shows a high friction coefficient and surface electrical resistance, so that its running durability is very poor. The latter method is disadvantageous in that the dispersing step requires much time and that characteristic properties of the ferromagnetic particles are lost, particularly the acicular shape of the particles is destroyed, resulting in worsened noise levels. Hence, either method is effective for obtaining all desired properties.
As an attempt to obtain a magnetic recording medium having a high loading of ferromagnetic particles and excellent electromagnetic characteristics such as sensitivity and suitable for high-density recording, while not impairing physical properties, JP-A-58-200425 proposes a magnetic recording medium comprising a second (upper) magnetic layer having incorporated therein a lubricating agent, an antistatic agent and an abrasive material, which serve to improve running durability, and a first (lower) magnetic layer which contains no such additives because the first larger does not significantly affect the running durability of the recording medium. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) This magnetic recording medium is good in S/N, head abrasion loss and still properties, but was found to be poor in running durability because the edges of the first magnetic layer are prone to suffer damage when the tape is run while the edges are in contact with parts of a tape recorder or the like.
On the other hand, JP-A-58-9215 proposes a magnetic recording medium comprising a surface magnetic layer containing no or a slight amount of inorganic oxide fine particles such as alumina particles and a lower magnetic layer containing such inorganic particles, for the purpose of diminishing the damage or abrasion loss of magnetic head which is caused when magnetic recording media are run while in contact with the magnetic head and, further, improving the durability of the magnetic recording medium. According to this JP-A, this recording medium although its duarability is improved is free from the problem that the presence of alumina in a surface magnetic layer results in alumina particles exposed on the surface of the magnetic layer and the exposed alumina particles cause magnetic heads to suffer damage or large abrasion loss. However, the magnetic recording medium of this JP-A shows very poor still durability, although it causes little head abrasion loss because the amount of an abrasive material in the second (upper) magnetic layer is small.